Referring now to FIG. 1a, fluid dynamic elements—in the context of the present invention—are three-dimensional bodies 1 comprising, in cross-section, first 3 and second 5 generally curved surfaces which extend in opposite directions from a leading edge 7 of the body 1 to meet at a trailing edge 9 of the body. The first and second surfaces have different curved profiles so that when the body is immersed in a fluid flow U at a positive angle of attack α, the fluid divides at the leading edge and flows at different speeds over each of the first and second surfaces. The difference in fluid velocities over the first and second surfaces induces, following Bernoulli's principle, a pressure difference that generates a force, which for aircraft wings is the lift force, commonly normalised as a dimensionless lift coefficient CL. The body also experiences a retarding force, called drag in the case of an aircraft wing, which can be normalised as a dimensionless drag coefficient, CD.
In the context of an aircraft, air flows smoothly over both surfaces of the body (wing) in normal flight, and for powered flight the thrust force applied (typically by the engines of the aircraft) for a given angle of attack must be sufficient to generate a lift that exceeds the drag. A stall can occur when a critical angle of attack is exceeded, and in this condition the airflow separates from the uppermost surface of the wing causing a dramatic loss of lift and a large increase in drag. This phenomenon of flow separation is often referred to in the art as a “boundary layer flow separation”, and is a phenomenon that is universally applicable to all types of elements that are capable of generating a fluid dynamic force when exposed to a fluid flow.
Boundary layer flow separation, namely the divergence of a flowing stream of fluid from a surface such as an aircraft helicopter wing or wind turbine rotor blades, can severely limit the operation, endurance, and performance of many engineering systems.
Boundary layer separation can be triggered and induced by several mechanisms. It can either be a natural consequence of the local flow and/or geometry, or be artificially induced by external disturbances and instabilities. On aero- or fluid- dynamic surfaces at high angle of attack to the oncoming air/fluid stream, the combination of adverse (increasing) pressure gradients from surface curvature, and the shear stresses between adjacent fluid layers and the surface, due to viscosity, can cause the air/fluid stream to separate from it. For aircraft wings and propeller and turbine rotor blades, flow separation results in a catastrophic loss of aerodynamic lift, a rapid increase in drag and a rapid increase in noise levels.
The development of techniques for the suppression, or delay to higher angle of attack, of flow separation on aircraft wings and rotor blades, has been a major research objective over the past 50 years. Flow separation due to surface curvature and viscosity can be delayed by the natural mixing in turbulent boundary layers. Turbulent flow embodies a relatively fast self-mixing and transport mechanism but it cannot transfer enough momentum into the boundary layer to maintain an attached flow in the presence of large adverse pressure gradients, such as on an aircraft wing or rotor blade pitched at high angle of attack.
Accordingly, and in order to prevent and delay the departure of the boundary layer from a surface, it has been proposed to utilise artificial flow mixing enhancement devices to re-energise the boundary layer. By re-energising the boundary layer to artificially increase the rate of fluid mixing within the boundary layer, one can increase the kinetic energy of the relatively low momentum near-surface fluid, and thereby delay to a higher angle of attack, or in some instances prevent the onset of flow separation.
Various flow control techniques, to reenergise boundary layers and thereby address flow separation, have been identified and successfully tested. For example, techniques such as slot blowing, tangential blowing, synthetic jets and vane vortex generators have previously been proposed. Of these, the method of increasing fluid mixing rates by the artificial generation of near surface longitudinal vortices has been found to be a particularly powerful technique. These vortices act to entrain high energy flow from an undisturbed outer fluid stream and transport it into a low momentum near-surface region deep inside the boundary layer. Mechanical, passive, vane vortex generators (first devised by Taylor, D. H., & Hoadley, H. H. and reported in “Application of vortex generator mixing principle to diffusers” Report R-15064-5, United Aircraft corporation, East Hartford, Conn., 1948) are the most common and widely used streamwise fluid vortex generators, and commonly consist of thin, protruding, solid strips fixed to the surface, usually located ahead of a region in which separated flow is likely to occur, at an angle to the oncoming flow.
However, whilst such devices resist the onset of flow separation it has been shown that mechanical vane type vortex generators also impose an increase in drag, caused by both the local pressure increase derived from the flow blockage by the device itself and by an increase in surface skin friction downstream of the device.
As an alternative to passive solid vane type vortex generators it has previously been proposed to provide an active fluid jet vortex-generating device (see Wallis, R. A., “The use of air jets for boundary layer control”, Aeronautical Research Laboratories, Australia, Aero. Note no. 110, 1952). The arrangement proposed by Wallis used fluid injection via inclined surface-bounded jets (more commonly known as active jet vortex generators or AJVGs) to induce longitudinal vortices for flow control.
Such AJVG systems usually consist of an array of small orifices, opening to a surface and supplied by a pressurised fluid source to induce longitudinal, or streamwise, vortices by virtue of the interaction between the fluid jets issuing from each orifice and fluid moving along the surface. AJVGs avoid the principal problem associated with passive vane type vortex generators in that they do not cause a large increase in drag. AJVGs can also be actively operated and controlled, depending on the flow characteristics over the surface.
Active vortex generating jets have been investigated as potential flow control devices for suppressing or at least postponing aerodynamic stall in fixed wing aircraft (see, for example, Innes, F., Pearcey, H. H., and Sykes, D. M., “Improvements in performance of a three element high lift system by application of air jet vortex generators”, The Aeronautical Journal, Vol 99, No 987, 1995), and more recently on rotating wing aircraft. AJVGs have proved to be successful in suppressing and delaying stall in laboratory wind tunnel experiments, and FIG. 1(b) is a graph illustrating the experimentally measured variation in lift coefficient (CL) with angle of attack (a) for an aerofoil wing section with and without AJVGs. The graph and accompanying schematic diagrams of illustrative aerofoils (at a 16° angle of attack) with and without AJVGs, show that flow separation (indicated by S in schematic) is delayed to higher angle of attack using AJVGs, and that consequently, a higher maximum value of CL can be achieved before stall.
However, whilst active vortex generating jets have been shown to provide performance enhancements, they have not yet been embraced by the engineering community, and passive vane type vortex generators are still commonly used on aircraft wings, despite their inherent drag increasing properties.
A likely explanation for this is that whilst AJVGs do provide performance advantages, they also require external energy input to generate the pressurised fluid for the fluid jets and the equipment required to provide this energy input greatly increases the overall weight of, for example, the aircraft. It is also the case that installation of an active system is inherently much more complex and hence costly than a simple vane vortex generator array.
It is apparent, therefore, that it would be highly advantageous if it were possible to design a fluid dynamic force generating element which avoided or at least reduced the increase in drag that characterises previously proposed vane-type passive vortex generators as well as the inherent disadvantages associated with previously proposed AJVGs, whilst at the same time providing comparable performance enhancements to those provided by the previously proposed AJVG systems.